Купальник, который я купила на распродаже, быстро высох, а плавки брата остались мокрыми до вечера.

Который is the relative pronoun meaning which / that. It introduces the clause который я купила на распродаже = which I bought on sale.

It appears as который because it agrees with купальник:

• купальник is masculine
• singular
• and in this clause it is the direct object of купила

So the pronoun has to be masculine singular accusative, which for an inanimate noun looks the same as the nominative: который.

If the noun were feminine, you would get a different form, for example:

• книга, которую я купила = the book that I bought

In the past tense, Russian verbs agree with the subject in gender and number.

Here the subject is я, but Russian past tense still shows the speaker’s gender:

• я купил = I bought, said by a man
• я купила = I bought, said by a woman

So купила tells you the speaker is female.

The expression на распродаже means at a sale / on sale / in a sale.

The preposition на here is used with the prepositional case:

• распродажа → на распродаже

This is a common fixed expression in Russian:

• купить на распродаже = to buy at a sale
• в магазине = in a store
• на рынке = at the market

So this is mostly vocabulary plus normal preposition + case usage.

Плавки is one of those Russian nouns that is normally used only in the plural, even when it refers to a single garment.

This is similar to English words like:

• shorts
• pants
• scissors

So плавки брата means my brother’s swim trunks / briefs, even if it is just one pair.

Because the noun is plural, related words also go into the plural:

• плавки ... остались
• мокрыми

Брата is in the genitive case because it shows possession.

So:

• плавки брата = the brother’s swim trunks / my brother’s swim trunks

This is a very common Russian way to express possession:

• книга сестры = sister’s book
• машина отца = father’s car
• дом друга = friend’s house

The basic form is брат, but after another noun to mean possession it becomes брата.

Russian uses two different ways to describe what happened:

• быстро высох = dried quickly / became dry quickly
• остались мокрыми = remained wet

So the first part focuses on a change of state:

• the swimsuit became dry

The second part focuses on no change:

• the trunks stayed wet

This contrast is very natural in Russian:

• one thing dried
• the other stayed wet

It also makes the sentence more expressive than simply repeating the same structure twice.

After verbs like быть, стать, оказаться, остаться, Russian often uses the instrumental case for the thing’s resulting or continuing state.

So:

• остались мокрыми = remained wet

The adjective мокрые changes to instrumental plural:

• nominative plural: мокрые
• instrumental plural: мокрыми

This happens because плавки is plural, and the adjective agrees with it.

Similar patterns:

• он стал врачом = he became a doctor
• она осталась спокойной = she remained calm
• они были счастливыми = they were happy

The preposition до requires the genitive case.

So:

• вечер → до вечера

До вечера means until evening.

Other examples:

• до утра = until morning
• до дома = up to the house / as far as the house
• до конца = until the end

So this is a case government rule: до + genitive.

That part is a relative clause, and in Russian these clauses are normally separated by commas.

So:

• Купальник, который я купила на распродаже, быстро высох

The commas mark extra information about купальник:

• which swimsuit?
• the one I bought on sale

Russian punctuation with relative clauses is generally more consistent than English punctuation here: if you have a clause with который, commas are usually required.

They are different kinds of swimwear.

• купальник usually means a swimsuit, often a woman’s one-piece or general bathing suit
• плавки usually means men’s swim trunks / briefs

So the sentence naturally contrasts:

• купальник belonging to the female speaker
• плавки брата belonging to her brother

Yes, высох is a perfective past form. It presents the drying as a completed result:

• it ended up dry

That works well with быстро:

• быстро высох = it dried quickly

If you used an imperfective verb, the focus would be different, more on the process than on the completed result.

In this sentence, perfective is natural because the point is the outcome:

• the swimsuit became dry
• the trunks did not; they stayed wet until evening

Yes, Russian word order is flexible, but the current order sounds natural and clear.

The structure is:

• Купальник — the topic
• который я купила на распродаже — extra identifying information
• быстро высох — what happened
• а плавки брата остались мокрыми до вечера — contrast with the second item

The conjunction а is especially important here. It means something like while / whereas / and on the other hand. It sets up a contrast:

• the swimsuit dried quickly
• the brother’s trunks stayed wet

You could move some adverbs around, but the given word order is very normal and idiomatic.

Not perfectly.

• а marks contrast or comparison between two facts
• и just means and
• но means but and sounds stronger, more oppositional

Here а is the best choice because the sentence compares two different outcomes:

• one item dried
• the other stayed wet

So а gives a natural contrast without sounding too dramatic.

Compare:

• Купальник высох, а плавки остались мокрыми = the swimsuit dried, whereas the trunks stayed wet
• Купальник высох, но плавки остались мокрыми = the swimsuit dried, but the trunks stayed wet

The second version is possible, but но feels a bit stronger.